1. Field of the Invention
The present invention relates to an image forming apparatus that includes a charging device for charging a photosensitive member.
2. Description of the Related Art
Recently, image forming apparatuses adopting a contact charging system that uses lower voltages and produces less ozone than a corona charger have been used. For example, an main body of an image forming apparatus that charges an image bearing member by use of a charging roller can be made more compact than that with a corona charger. To charge a photosensitive member serving as an image bearing member by use of a charging roller, a voltage in which AC (alternating current) and DC (direct current) voltages overlap is applied to the charging roller. Thereby, the photosensitive member is uniformly charged. An “alternating current charging system” in which an image bearing member is charged by a voltage in which alternating current and direct current voltages overlap can charge the image bearing member more uniformly than a “direct current charging system” in which an image bearing member is charged by a direct current voltage only. However, the “alternating current charging system” is greater than the “direct current charging system” in the quantity of discharge to the image bearing member. A large quantity of discharge increases the production of discharge products, which are factors of image flow and wear to an image bearing member. In view of the foregoing problem, Japanese Patent Application Laid-Open No. 2001-201920 discloses a method in which while uniform charging is ensured by the “alternating current charging system,” an alternating current voltage (peak-to-peak value) is set for minimizing the required quantity of discharge.
However, the relation between the voltage and the quantity of discharge changes according to the thickness of photosensitive and dielectric layers of an image bearing member, changes in ambient air, or type of charging member. For example, in a low temperature and low humidity environment (15° C. temperature and 10% or less humidity, hereinafter referred to as L/L environment), a material dries and the resistance increases, making it difficult to generate a discharge. Conversely in a high temperature and high humidity environment (at least 30° C. temperature and at least 80% humidity, hereinafter referred to as H/H environment), a material absorbs moisture and resistance decreases, making it easier to generate a discharge. If the alternating current voltage of a peak-to-peak value suitable for an L/L environment is applied to a charging roller in an H/H environment, the quantity of discharge increases beyond the necessary level. Increases in the quantity of discharge lead to “increases in wear to image bearing members,” “image flow (blurring of electrostatic latent images) caused by discharge products,” or “toner fusion.”
To overcome the foregoing problems, the quantity of discharge current has been controlled for a predetermined number of sheets in order to ensure appropriate charging regardless of the environmental changes. Next, discharge current control as disclosed in Japanese Patent Application Laid-Open No. 2001-201920 will be briefly described.
In conventional discharge current control, the peak-to-peak value of an alternating current voltage applied to a charging member is adjusted. Then, the overall current flowing between the charging member and the image bearing member at several points in an undischarged area and at several points in a discharged area are measured. After a discharge initiating point is calculated from the measurement result, a peak-to-peak value is controlled based on the relation between the peak-to-peak value of the alternating current voltage and the amount of discharge current, so as to produce an appropriate quantity of discharge current (see FIG. 9).
In a conventional control of discharge current, an appropriate quantity of discharge current is ensured immediately after the control of the quantity of discharge current. However, this quantity of discharge deviates from the appropriate level before the subsequent control of discharge current. To avoid this, the quantity of discharge current is frequently controlled so as to minimize any difference from an appropriate quantity of discharge current. However, in a conventional control of the quantity of discharge current, in which the peak-to-peak value of the alternating current voltage is adjusted and the overall current is measured at a number of points, entails a longer control time, which decreases productivity.